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Fruit flies can either have a brown body or a black body. The allele for a brown body (B) is dominant to the allele for a black body (b).

A brown fruit fly was crossed with a black fruit fly as shown below. What is the probability of a resulting offspring being brown?
B B
b ? ?
b ? ?
A.
50%
B.
75%
C.
25%
D.
100%

1 answer

To determine the probability of the offspring being brown when a brown fruit fly is crossed with a black fruit fly, we first need to identify the genotypes of the parent flies.

  • The brown fruit fly (which has the dominant allele for brown body) can either be homozygous (BB) or heterozygous (Bb). However, since it is crossed with a black fruit fly (bb), we must consider the two possible cases:

  1. If the brown fruit fly is homozygous (BB):

    • Possible gametes from brown fly: B, B
    • Possible gametes from black fly: b, b
    • Resulting offspring genotypes: All Bb (brown)

  2. If the brown fruit fly is heterozygous (Bb):

    • Possible gametes from brown fly: B, b
    • Possible gametes from black fly: b, b
    • Resulting offspring genotypes:
      • Bb (brown) and bb (black)

To find the proportion of brown offspring in the heterozygous case:

  • Possible offspring:
    • Bb (brown): 1
    • bb (black): 1

So, the ratio of brown offspring to total offspring would be 1 out of 2, or 50%.

Since the problem does not specify whether the brown fly is homozygous or heterozygous, we assume the heterozygous case is the likely scenario in typical genetics problems unless otherwise noted.

Therefore, if the brown fruit fly is heterozygous (Bb), the probability of an offspring being brown is:

A. 50%.